1. Technical Field
The present invention relates to a vertical lifting tube suspended at its upper part and provided at its lower part with one or more suction cups. The lifting tube is connected to a vacuum source and is axially extensible and contractible depending upon the magnitude of the current air pressure in the lifting tube. The pressure in the lifting tube is manually adjustable by means of valve operated by a regulating means.
2. Description of the Prior Art
Hoisting devices of this type are known for instance by SE 451 834, and are provided with a valve device at the lower part of the lifting tube by means of which surrounding air, when required, can be let into the interior of the lifting tube such that the air pressure in the lifting tube can be regulated to permit lifting and lowering of the lower part of the lifting tube in a desired manner. This results in an extremely simple mode of operating of the lifting device by holding an object by suction, lifting the suction device and the object, lowering same, and delivering of the object only by the aid of the valve device.
A drawback to this known type of hoisting device is that the vacuum device, a vacuum pump or an ejector device driven by compressed air, is operated all the time with maximum vacuum flow that is dimensioned for the heaviest expected load and for fast lifting operations. As soon as the lifting operation is finished after 1-2 seconds, the demand of vacuum flow is essentially reduced in dependence on how much leakage there is through the object that is lifted and around the edges of the suction device. This means that it is necessary to let in air into the lifting tube corresponding to the over-capacity of the vacuum source. If this is not done the vacuum level in the lifting tube will increase and the load will rise to maximum lifting height which is not desirable. All the air let in generates a completely unnecessary pumping work which calls for unnecessarily strong dimensioning of the pump and raises the manufacturing costs as well as the operating costs. Vacuum flows of this magnitude normally also make it necessary to use fans and make it impossible to use simple and easily applicable ejectors driven by compressed air.